This invention relates to subsea wellhead systems and more particularly, to methods and apparatus for supporting, holding down, and sealing casing hangers within a subsea wellhead.
Increased activity in offshore drilling and completion has caused an increase in working pressures such that it is anticipated that new wells will have a working pressure of as high as 15,000 psi. To cope with the unique problems associated with underwater drilling and completion at such increased working pressures, new subsea wellhead systems are required. Wells having a working pressure of up to 15,000 psi are presently being drilled off the coast of Canada and in the North Sea in depths of over 300 feet. These drilling operations generally include a floating vessel having a heave compensator for a riser and drill pipe extending to the blowout preventer and wellhead located at the mud line. The blowout preventer stack is generally mounted on 20 inch pipe with the riser extending to the surface. A quick disconnect is often located on top of the blowout preventer stack. An articulation joint is used to allow for vessel movement. Two major problems arise in 15,000 psi working pressure subsea wellhead systems operating in this environment, namely, a support shoulder in the wellhead housing which will support the casing and pressure load, and a sealing means between the casing hangers and wellhead which will withstand and contain the working pressure.
In the past, prior art wellhead designs permitted adequate landing support for successive casing hangers. However, with the increase in pressure rating and the landing and supporting of multiple casing strings and tubing strings within the wellhead, a small support shoulder will not support the load. Although an obvious answer to the problem would be to merely use a support shoulder large enough to support the casing and pressure load, large support shoulders projecting into the flow bore in the wellhead housing restrict access to the casing below the wellhead housing for drilling. In the early days of offshore drilling, 163/4 inch bore subsea wellhead systems required underreaming. At that time, most floating drilling rigs were outfitted with a 163/4 inch blowout preventer system to eliminate the two stack (20 inch and 135/8 inch) and the two riser system required up until that time. As wellhead systems moved from 5,000 psi to 10,000 psi working pressure, the 183/4 inch, 10,000 psi support shoulder was developed to carry casing and pressure loads and to provide full access into the casing below the wellhead housing.
The second major problem is the sealing means. The sealing means must be capable of withstanding and containing 15,000 psi working pressures. Available energy sources for energizing the sealing means include weight, hydraulic pressure, and torque. Each sealing means requires different amounts of energy to position and energize. Weight is the least desirable because the handling of drill collars providing the weight is difficult and time consuming on the rig floor. If hydraulic pressure is applied through the drill pipe, there is a need for wireline equipment to run and recover darts from the hydraulic-to-actuated seal energization system. If darts are not used, the handling of "wet strings" of drill pipe is very messy and unpopular with drilling crews. If the seal energization means uses the single trip casing hanger technique, the cementing fluid can cause problems in the hydraulic system used to energize the seal. Maintenance is also a problem. Although torque is the most desirable method to energize a seal, there are limitations on the amount of torque which can be transmitted from the surface due to friction losses to riser pipe, the blowout preventer stack, off location, various threads, and the drill pipe itself.
The subsea wellhead system of the present invention overcomes the deficiencies of the prior art and includes many other advantageous features. The system is simple, has less than 50 parts and is suitable for H.sub.2 S service. The system has single trip capability but can still use multiple trip methods. All hangers are interchangeable with respect to the outer profile so that they can be run in lower positions. The seal elements are interchangeable and are fully energized to a pressure in excess of the anticipated wellbore pressure. Back-up seals are available. The seals are not pressure de-energized. The hangers can be run without lock downs and the seal elements will seal even if the hanger lands high.
The housing support seat supports in excess of 6,000,000 lbs. (working pressure plus casing weight or test pressure) without exceeding 150% of material yield in compression. The wellhead will pass a 171/2 inch diameter bit. The present invention does not attempt to land on two types of seats at once or on two seats at once. Further, the housing support seat is not sensitive to collecting trash during drilling or to collecting trash during the running of a 133/8 inch casing. Further, the housing support seat does not require a separate trip nor does it drag snap rings down the bore.
The hanger hold down will hold down 2,000,000 lbs. The hanger hold down is positively mechanically retracted when retrieving the casing hanger body and is compatible with single trip operations. The hanger hold down is released for retrieval of the casing hanger when the seal element is retrieved. The hanger hold down is compatible with multiple trip operations and permits the running of the hanger with or without the hold down. The sealing means will work even if the hold down is not used. The hanger hold down is reusable and has a minimum number of tolerances to stack up between hold down grooves.
The sealing means of the present invention will reliably seal an annular area of approximately 181/2 inch outside diameter by 17 inch inside diameter and provide a rubber pressure in excess of 15,000 psi (20,000 psi nominally) when the sealing means is energized and the sealing means sees a pressure from above or below of 15,000 psi. The pressure in excess of 15,000 psi is retained in the sealing means after the running tool is removed. The sealing means is additionally self-energized to hold full pressure where full loading force was not applied or where full loading force was not retained. The sealing means will not be pressure de-energized. The sealing means provides a relatively long seal area to bridge housing defects and/or trash. Further, the sealing means provides primary metal-to-metal seals and uses the metal-to-metal seals as backups to prevent high pressure extrusion of secondary elastomeric seals. The sealing means of the present invention positively retracts the metal-to-metal seals from the walls prior to retrieving the sealing means. The elastomeric seals of the sealing means are allowed to relax during retrieval of the packoff assembly and is completely retrievable. The present sealing means provides a substantial metallic link between the top and the bottom of the packing seal area to insure that the lower ring is retrievable. The design allows for single trip operations. There are no intermittent metal parts in the seal area to give irregular rubber pressures. The sealing means provides a minimum number of seal areas in parallel to minimize leak paths. The sealing means is positively attached to the packing element so that it cannot be washed off by flow during the running operations. The design also allows for multiple trip operations and is interchangeable for all casing hangers within a nominal size.
The means to load the sealing means reliably provides a force to energize the sealing means to a nominal 20,000 psi. It allows full circulation if used in a single trip. However, the loading means is compatible with either a single trip operation or multiple trip operation. Further, it is interchangeable for all casing hangers within the wellhead system. The loading means will cause the sealing means to seal even if the casing hanger is set high. Further, it does not release any significant amount of the full pressure load after actuation. The loading means does not require a remote engagement of hold down threads. Further, it has no shear pins. The loading means is reusable and does not have to remotely engage hold down threads on packing nut replacement.
The casing hanger running tool includes a connection between the running tool and casing hanger which will support in excess of 700,000 lbs. of pipe load. The running tool is able to generate an axial force in excess of 900,000 lbs. to energize the sealing means. Further, the running tool is able to tie back into the casing hanger without a left hand torque. The running tool can be run on either casing or drill pipe.
Other objects and advantages of the invention will appear from the following description.